One way to think about workplace performance is in terms of a person’s gas tank.

The idea is that employees have a reservoir of physical and psychological resources for fueling their work contributions. Things like vacations, support from others and job autonomy can add to the reservoir, while difficult bosses, financial anxiety, job insecurity and heavy workloads can drain it.

“When fuel levels reach unhealthy levels, additional pressures are often debilitating, with impaired performance as one of the first consequences,” says Wayne Hochwarter, a management professor at Florida State University in Tallahassee.

Hochwarter points to research he did about job stress and employee effort in the wake of hurricanes. Hurricane-related work stress—such as demands to work extra hours to make up for absent colleagues—had a negative effect on employees whose resources prior to the storms already had been taxed by pressures such as the inability to get help at work. Those employees, whose gas tanks effectively had been running low, said hurricane-related stress led to a decline in their work efforts, Hochwarter says. On the other hand, employees with “adequate resource levels” leading up to the hurricanes were more engaged at work when storm-related demands were high.

Workplace scholar Dave Ulrich makes a similar point. “With increased demands—workload—employees need increased resources to stay in ‘balance’ or equilibrium,” says Ulrich, who is a professor at the Ross School of Business at the University of Michigan. “When employees have more resources—control of their time, opportunities to learn and grow, etcetera—they can cope with increased demands. Stress occurs when demands exceed resources.”

A number of observers see worker tanks nearing the empty line.

Among them are management professor John Boudreau and former Northrop Grumman Corp. chief human resources officer Ian Ziskin. “Employees are, to put it simply, just plain exhausted—physically, emotionally and psychologically,” Boudreau and Ziskin write in a recent issue of the journal Organizational Dynamics. They also argue that spent employees are a real risk for businesses in the long run: “Burnt out, fatigued employees are unable to build sustainable performance.”

Hochwarter’s research during the past year shows that employees in a demanding work environment said their job performance had risen. But their anxiety levels at work and home also rose while their job satisfaction fell. That’s a troubling sign, according to Hochwarter’s research.

He says spikes in productivity sparked by new or increased demands are not sustainable unless employee tanks are filled with new resources. “Without doing so, you are going to see half-tanks turn into tanks with only fumes,” he says.